Technical Field
The present disclosure relates to a back-side illuminated image sensor, that is, an image sensor comprising pixels formed inside and on top of a semiconductor substrate having a first surface, called back side, intended to receive an illumination, and having a second surface, called front side, topped with an interconnection structure, components such as transistors being formed on the front side. The present disclosure also relates to a pixel of such a sensor.
Description of the Related Art
FIG. 1 is a copy of FIG. 1 of U.S. Pat. No. 8,513,761 of the applicant and schematically illustrates an example of a circuit of an image sensor pixel.
The pixel comprises a photodiode D associated with a charge transfer transistor TR, a precharge device, and a read device. The precharge device is formed of an N-channel MOS transistor RST, interposed between a power supply rail Vdd and a sense node S. The read device is formed of the series connection of N-channel MOS transistors SF and RD. The drain of transistor SF is connected to power supply rail Vdd. The source of transistor RD is connected to an input terminal P of a processing circuit (not shown). The gate of read transistor SF, assembled as a source follower, is connected to sense node S. The anode of photodiode D is connected to reference power supply rail GND and the cathode of photodiode D is connected to node S via transistor TR. Generally, the gate control signals of transistors RD, RST, and TR are provided by control circuits, not shown in FIG. 1, and may be provided to all the pixels of a same row of a pixel array. It should be noted that photodiode D is always associated with a transistor TR, but that there may exist a single precharge device and a single read device for a group of photodiodes, the drains of the transfer transistors of these photodiodes then being interconnected to a same node S.
FIG. 2 is a copy of FIG. 5 of U.S. Pat. No. 8,513,761 of the applicant and schematically illustrates an embodiment of a pixel of the type in FIG. 1, FIG. 2 being a cross-section view of this pixel.
The pixel comprises a lightly-doped N-type silicon substrate 11 (N−). An insulating structure comprising an insulated trench (insulator 23) filled with a conductive material 24 connected to a terminal Vwall surrounds the pixel and separates it from the adjacent pixels. A heavily-doped P-type layer 13 (P+) coats substrate 11 on the side of the upper surface, or front side, of the pixel. Layer 13 is interrupted in a substantially central portion of the pixel by a transfer region comprising a lower light-doped N-type portion 17 (N−) and an upper heavily-doped N-type portion 18 (N+). An insulated trench (insulator 15) filled with a conductive material 16 connected to a terminal TG laterally delimits the transfer region. In other words, in top view, not shown, the transfer region is surrounded with insulating structure 15, 16, which is ring shaped, insulating structure 15, 16 being itself surrounded with layer 13. Layer 13 has various transistors, for example, transistors RD, RST, and SF of FIG. 1 (transistor RD is not shown), formed therein. Various metallization levels are formed on the upper surface, or front side, of substrate 11 to provide the connections to the drains, sources, and gates of the various transistors and to N+ region 18 corresponding to sense node S of FIG. 1. A heavily-doped P-type layer 19 (P+) is arranged at the level of the lower surface, or back side, of substrate 11. Further, the lower surface or back side of the pixel is covered with a filter 20 having the desired color for the considered pixel, for example, red, green, or blue. In this example, a lens 21 covers filter 20.